Phase detector for reducing noise

ABSTRACT

The present invention provides a method and an apparatus for reducing noise. The apparatus includes a phase detector adapted to determine a phase difference between a first and a second signal, a first circuit adapted to generate a control signal based upon the determined phase difference, and a second circuit. The second circuit is adapted to receive a third signal, receive a fourth signal, modify the fourth signal based upon the control signal, and provide the third signal and the modified fourth signal to the phase detector as the first and second signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the synchronization of periodicsignals, such as clock signals. More particularly, the invention relatesto a phase detector for reducing noise.

2. Description of the Related Art

Many high speed electronic systems possess critical timing requirementsthat dictate the need to generate a periodic clock waveform possessing aprecise timing relationship with respect to some reference signal. Theimproved performance of computing integrated circuits and the growingtrend to include several computing devices on the same board present achallenge with respect to synchronizing the time frames of all thecomponents.

While the operation of various components in the system should be highlysynchronized, i.e., the maximum skew in time between the significantedges of the internally generated clocks of the components should beminimized, it is not enough to feed the reference clock of the system tothese components. This is because different chips may have differentmanufacturing parameters, which, when taken together with additionalfactors such as ambient temperature, voltage, and processing variations,may lead to large differences in the phases of the respective chipgenerated clocks. Conventionally, synchronization is achieved by usingdigital delay locked loop (DDLL) circuits to detect the phase differencebetween clock signals of the same frequency and produce a digital signalrelated to the phase difference. By feeding back the phasedifference-related signal to control a delay line, the timing of oneclock signal is advanced or delayed until its rising edge is coincidentwith the rising edge of a second clock signal.

The phase differences are typically detected by a phase detectorincluded in the digital delay locked loop circuit. The phase detectorcompares an input, or reference, signal to a feedback signal and thengenerates a signal proportional to the phase difference between thereference and feedback signals. The signal proportional to the phasedifference is then provided to a so-called “delay line” that mayintroduce a variable delay into the delay path of the input signal.Conventional digital delay lines include a number of discrete delayelements, each of which may be capable of introducing a single unitdelay into the delay path. For example, a conventional delay element maybe an inverter that introduces a unit delay of about 200 picosecondsinto the delay path. Thus, the phase detector signal typicallycorresponds to a series of “up” and “down” signals that indicate that adelay element should be introduced into, or removed from, the delaypath, respectively.

When the phase difference between the input signal and the referencesignal becomes close to, or less than, one unit delay, conventionalphase detectors generate a series of alternating “up” and “down”signals. If several clock cycles are required for a signal to propagatethrough the delay path and feedback to the phase detector, the series ofadjustments indicated by the alternating “up” and “down” signals maycreate noise that may cause loop stability problems in the digital delaylocked loop. The series of adjustments performed in response to thealternating signals may also cause the phase of the output clock signalto advance and retreat on successive clock cycles, creating noise, suchas jitter, in the output clock signal. Noise in the output clock signalmay reduce the stability of, or cause an error in, the digital devicerelying on the output clock.

SUMMARY OF THE INVENTION

In one aspect of the instant invention, an apparatus is provided forreducing noise. The apparatus includes a phase detector adapted todetermine a phase difference between a first and a second signal, afirst circuit adapted to generate a control signal based upon thedetermined phase difference, and a second circuit. The second circuit isadapted to receive a third signal, receive a fourth signal, modify thefourth signal based upon the control signal, and provide the thirdsignal and the modified fourth signal to the phase detector as the firstand second signals.

In another aspect of the present invention, a method is provided forreducing noise. The method includes receiving a clock signal, receivinga feedback signal formed using the clock signal, generating a controlsignal indicative of a phase difference between the clock signal and thefeedback signal, and modifying the feedback signal based upon thecontrol signal.

In another aspect of the instant invention, a delay locked loop isprovided. The delay locked loop includes an adjustable delay elementadapted to receive an input clock signal and provide an output clocksignal, a control logic coupled to the adjustable delay element, and aphase detector communicatively coupled to the control logic. The phasedetector is adapted to determine a phase difference between the inputclock signal and the output clock signal and includes a hysteresisadjuster adapted to provide a modified output clock signal based uponthe output clock signal and a control signal, a phase detector coreadapted to determine a phase difference between the input clock signaland the modified output clock signal, and a digital filter adapted toprovide the control signal to the hysteresis adjuster based upon thephase difference.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

FIG. 1 is a simplified block diagram of a digital system having a delaylocked loop circuit;

FIG. 2 shows one embodiment of a delay locked loop circuit that may beused in the digital system shown in FIG. 1;

FIG. 3 shows one embodiment of a phase detector that may be used in thedelay locked loop circuit shown in FIG. 2;

FIG. 4 shows one embodiment of a hysteresis adjuster that may be used inthe phase detector shown in FIG. 3; and

FIGS. 5A and 5B show simulated input and output signals for a phasedetector without a hysteresis adjuster and with a hysteresis adjuster,respectively.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

FIG. 1 shows a simplified block diagram of a digital system 100. Thedigital system 100 includes a digital device 105 coupled to a seconddigital device 110. The digital device 105 provides a reference clocksignal (CLKIN) to the second digital device 110. The second digitaldevice 110 uses the CLKIN signal to synchronize its internal clocksusing a delay locked loop 115 and generate an output clock signal(CLKOUT). As an illustrative example, the second digital device 110 maybe a memory device that synchronizes its output data on a data line 120with the CLKOUT signal. Although the present invention will be describedin the context of the delay locked loop 115, persons of ordinary skillin the art will appreciate that the present invention may be used inother loops, including phase locked loops and the like.

FIG. 2 illustrates one embodiment of the delay locked loop 115 thatreceives the CLKIN signal and provides the CLKOUT signal. In theillustrated embodiment, the CLKIN signal and a FEEDBACK signal areprovided to a phase detector 200, which may determine a phase differencebetween the CLKIN signal and the FEEDBACK signal. In one embodiment, theFEEDBACK signal is approximately equal to the CLKOUT signal. However,the present invention is not so limited. In one alternative embodiment,additional delays, such as input/output delays and the like, may causethe FEEDBACK signal and the CLKOUT signal to have a different phase.

The phase detector 200 provides one or more signals indicative of thephase difference between the CLKIN signal and the FEEDBACK signal to acontrol logic 210. In the illustrated embodiment, the phase detector 200provides a binary output based on the relative phase difference betweenthe CLKIN and FEEDBACK signals. For example, the phase detector 200 mayprovide a shift-left signal (SL) if the FEEDBACK signal leads the CLKINsignal and a shift-right signal (SR) if the FEEDBACK signal lags theCLKIN signal.

In the illustrated embodiment, the control logic 210 includes a shiftregister 215 that may receive the shift-left signal (SL) or theshift-right signal (SR). However, it will be appreciated by those ofordinary skill in the art that the shift register 215 is not necessaryfor the practice of the present invention, and thus the control logic210 may not include the shift register 215. Furthermore, the presentinvention is not limited to phase detectors 200 that provide binaryoutput. In various alternative embodiments, the phase detector 200 mayprovide any desirable signal to indicate the relative phase differencebetween the CLKIN and the FEEDBACK signals.

The CLKIN signal is also provided to an adjustable delay 220 in thedelay locked loop 115. In one embodiment, the adjustable delay 220includes a plurality of delay elements 225(1–n). However, the number ofdelay elements 225(1–n) is a matter of design choice and thus, unlessexpressly set forth in the claims, the present invention is not limitedto any specific number of delay elements 225(1–n). Each of the delayelements 225(1–n) may be capable of introducing a unit time delay intothe CLKIN signal. For example, the delay elements 225(1–n) may each be adigital device, such as a NAND gate (not shown) and the like, that mayintroduce a unit time delay of about 200 picoseconds into the CLKINsignal. However, it will be appreciated by those of ordinary skill inthe art that, in alternative embodiments, the delay elements 225(1–n)may not be digital devices. Furthermore, the adjustable delay 220 maynot be formed of a plurality of separate delay elements 225(1–n). Forexample, in various alternative embodiments, the adjustable delay 220may include one or more analog devices capable of introducing a variabletime delay into the CLKIN signal.

The delay locked loop 115 may also include a delay 230. For the sake ofclarity, the delay 230 is represented in FIG. 2 as a single elementcoupled to the adjustable delay 220. However, those of ordinary skill inthe art will appreciate that the delay 230 may not be a single element.In particular, the delay 230 is intended to be representative of thevarious delays, such as clock distribution delays and the like, that maybe introduced into the CLKIN signal by the devices and lines within thedelay locked loop 115. For example, the delay locked loop 115 may havean intrinsic delay, represented in FIG. 2 as the delay 230, ranging fromabout 1 clock cycle to up to about 8 clock cycles, depending on theprocess, the voltage, the period of the CLKIN signal and/or thetemperature. However, as mentioned above, the delay 230 may or may notinclude additional delays, such as input and/or output delays, which mayaffect the CLKIN and CLKOUT signals.

The control logic 210 is coupled to the adjustable delay 220. In theillustrated embodiment, the control logic 210, may provide a signal tothe adjustable delay 220 that may be used to select one or more of thedelay elements 225(1–n). An adjustable delay 220 may be introduced intothe CLKIN signal by providing the CLKIN signal to the one or moreselected delay elements 225(1–n). In one embodiment, the control logic210 may select one or more of the delay elements 225(1–n) to compensatefor the delay 230 such that the CLKIN signal is approximately in phasewith the CLKOUT signal. For example, if the delay locked loop 115, hasan intrinsic delay of about 3.5 nanoseconds and the period of the CLKINsignal is about 5 nanoseconds, then seven delay elements 225(1–n), eachhaving a unit time delay of about 200 picoseconds, may be selected bythe control logic 210. In the illustrated embodiment, the control logic210 may select the one or more delay elements 225(1–n) using informationstored in the shift register 215. However, as discussed above, the shiftregister 215 is not necessary for the practice of the present invention.

The phase detector 200 may detect a phase difference between the CLKINsignal and the FEEDBACK signal that is less than about one unit timedelay, which may result in the phase detector 200 providing anadjustment signal, such as a shift-left signal (SL) and/or a shift-rightsignal (SR), to the control logic 210. The adjustable delay 220 mayover-adjust the CLKIN signal by introducing a time delay of at least oneunit time delay into the CLKIN signal. The over-adjusted CLKIN signalmay be provided to the phase detector 200 via the FEEDBACK path, whichmay detect the new phase difference between the CLKIN signal and theFEEDBACK signal and provide a compensating adjustment signal to thecontrol logic 210. However, the FEEDBACK signal may be delayed byapproximately the delay 230. Thus, to reduce the number of undesirableadjustment signals that may be provided to the control logic 210, andthereby reduce noise and improve the loop stability of the delay lockedloop 115, a CONTROL signal may be used in one embodiment of the phasedetector 200, as illustrated in FIG. 3.

In the illustrated embodiment, the phase detector 200 includes ahysteresis adjuster 300, a phase detector core 310, and a digital filter320. The hysteresis adjuster 300, which is described in detail below,receives the CLKIN signal and the FEEDBACK signal and provides modifiedsignals, CLKIN′ and FEEDBACK′, to the phase detector core 310. In oneembodiment, the phase detector core 310 may be a latch-type phasedetector. However, the specific type of phase detector core 310 is amatter of design choice and therefore the present invention is notlimited to latch-type phase detectors. In various alternativeembodiments, any desirable type of phase detector 200, includingarbiter-type phase detectors, counter-type phase detectors, and thelike, may be used to form the phase detector core 310.

The phase detector core 310 generates a signal that is indicative of thephase difference between the CLKIN′ signal and the FEEDBACK′ signal. Inthe illustrated embodiment, the phase detector core 310 generates an UPsignal to indicate that the CLKIN′ signal leads the FEEDBACK′ signal anda DOWN signal to indicate that the CLKIN′ signal lags the FEEDBACK′signal. The phase detector core 310 provides the UP and DOWN signal tothe digital filter 320, which may use the UP and DOWN signals torespectively generate the shift-left signal (SL) and the shift-rightsignal (SR). In one embodiment, the digital filter 320 may be a majorityfilter that may provide a signal in response to detecting at least twoconsecutive determined phase differences in the same direction. Forexample, the digital filter 320 may provide a shift-left signal (SL) inresponse to receiving two consecutive UP signals and a shift-rightsignal (SR) in response to receiving two consecutive DOWN signals.However, those of ordinary skill in the art will appreciate that, inalternative embodiments, the digital filter 320 may provide theshift-right signal (SR) and/or the shift-left signal (SL) in response toreceiving any desirable sequence of UP and/or DOWN signals.

The digital filter 320 also provides a CONTROL signal to the hysteresisadjuster 300. In one embodiment, the CONTROL signal is not filtered bythe digital filter 320. For example, the CONTROL signal may correspondto the UP and/or DOWN signals provided to the digital filter 320 by thephase detector core 310. However, in alternative embodiments, theCONTROL signal may be any desirable signal provided by the digitalfilter 320. The hysteresis adjuster 300 may provide modified signals,CLKIN′ and FEEDBACK′, in response to receiving the CONTROL signal, asdescribed below.

FIG. 4 shows one embodiment of the hysteresis adjuster 300, whichincludes a fixed delay 400 and an adjustable delay 410. In oneembodiment, the fixed delay 400 forms the CLKIN′ signal by introducing apredetermined time delay into the CLKIN signal. For example, the fixeddelay 400 may include two delay elements (not shown) to introduce apredetermined time delay of about two unit delays into the CLKIN signal.However, as will be appreciated by those of ordinary skill in the art,the predetermined time delay is not material to the present invention.In various alternative embodiments, any desirable delay may beintroduced into the CLKIN signal by the fixed delay 400.

The adjustable delay 410 may introduce a selected time delay into theFEEDBACK signal to form the FEEDBACK′ signal. In one embodiment, thetime delay is selected in response to receiving the CONTROL signal. Ifthe CONTROL signal corresponds to the DOWN signal, the selected timedelay may be larger than the time delay introduced by the fixed delay400. For example, if the fixed delay 400 introduced a time delay ofabout two unit delays, the adjustable delay 410 may introduce a selectedtime delay of about three unit delays into the FEEDBACK signal. If theCONTROL signal corresponds to the UP signal, a selected time delay maybe about equal to the time delay introduced by the fixed delay 400. Forexample, if the fixed delay 400 introduced a time delay of about twounit delays, the adjustable delay 410 may also introduce a selected timedelay of about two unit delays into the FEEDBACK signal.

To illustrate the operation of the phase detector 200, consider thefollowing example. Initially, the CLKIN signal is assumed to lag theFEEDBACK signal by less than about one unit time delay. The fixed delay400 and the adjustable delay 410 are further assumed to introduce equaltime delays into the CLKIN signal and the FEEDBACK signal, respectively.Thus, the CLKIN′ signal initially lags the FEEDBACK′ signal by less thanabout one unit time delay. The phase detector core 310 then determinesthat the CLKIN′ signal lags the FEEDBACK′ signal and provides a DOWNsignal to the digital filter 320. As discussed above, in thisembodiment, the digital filter 320 is a majority filter. Assuming thatthis is the first DOWN signal received by digital filter 320, noshift-left signal (SL) or shift-right signal (SR) is provided by thedigital filter 320, and the adjustable delay 220 shown in FIG. 2 remainsunchanged.

The digital filter 320 also provides the CONTROL signal indicative ofthe DOWN signal to the hysteresis adjuster 300. In response to receivingthe CONTROL signal indicative of the DOWN signal, the adjustable delay410 introduces a delay into the FEEDBACK signal that is larger than thedelay introduced into the CLKIN signal by the fixed delay 400. Forexample, if the delay introduced by the fixed delay 400 is approximatelytwo unit delays, then the adjustable delay 410 will introduce a delay ofapproximately three unit delays in response to receiving the CONTROLsignal indicative of the DOWN signal.

The delay introduced by the adjustable delay 410 results in the CLKIN′signal leading the FEEDBACK′ signal by less than about one time delayunit during the next clock cycle, assuming that the phase differencebetween the CLKIN signal and the FEEDBACK signal remained approximatelyconstant. Thus, the phase detector core 310 provides an UP signal to thedigital filter 320. Since the previous signal received by the digitalfilter 320 was a DOWN signal, no shift-left signal (SL) or shift-rightsignal (SR) is provided by the digital filter 320, and the adjustabledelay 220 shown in FIG. 2 remains unchanged.

In response to receiving the CONTROL signal indicative of the UP signal,the adjustable delay 410 reduces the delay introduced into the FEEDBACKsignal so that is about equal to the delay introduced into the CLKINsignal by the fixed delay 400. For example, if the delay introduced bythe fixed delay 400 is approximately two unit delays, then theadjustable delay 410 will introduce a delay of approximately two unitdelays in response to receiving the CONTROL signal indicative of the UPsignal. Consequently, the CLKIN′ signal will now lag the FEEDBACK′signal by less than one time delay unit during the next clock cycle,assuming that the phase difference between the CLKIN signal and theFEEDBACK signal remained approximately constant.

As illustrated by the previous example, differences between the CLKINsignal and the FEEDBACK signal that are less than about one unit timedelay may not result in a shift-left signal (SL) or a shift-right signal(SR) being provided by the digital filter 320, so that the adjustabledelay 220 shown in FIG. 2 remains unchanged. The loop stability of thephase detector 200 may therefore be increased by including thehysteresis adjuster 300, the phase detector core 310, and the digitalfilter 320. Furthermore, by increasing the loop stability of the phasedetector 200 in the manner described above, undesirable jitter in theoutput clock signal may be reduced, thereby increasing the stability ofthe digital devices 105, 110 relying on the output clock and reducingerrors that may be caused by jitter of the output clock signal.

Illustrative input and output signals for two embodiments of the phasedetector 200 are shown in FIGS. 5A and 5B. It will be appreciated bythose of ordinary skill in the art that the illustrated signals areexemplary in nature and are not intended to limit the present invention.FIG. 5A shows two simulated input signals 500 having a small phasedifference. It will be appreciated that the coarse resolution of FIG.5A, and the small phase difference between the two input signals 500,causes the two input signals 500 to blend together so that they appearas a single line. FIG. 5A also shows a simulated output signal 505 for aphase detector, such as the phase detector 200 shown in FIG. 2, withouta hysteresis adjuster 300 or a filter, such as the hysteresis adjuster300 and the digital filter 320 shown in FIG. 3. A simulated outputsignal 510 for a phase detector core, such as the phase detector core310 shown in FIG. 3, is also shown in FIG. 5A.

FIG. 5B shows two simulated input signals 520 having a small phasedifference. It will be appreciated that the coarse resolution of FIG.5B, and the small phase difference between the two input signals 520,causes the two input signals 520 to blend together so that they appearas a single line. FIG. 5B also shows a simulated output signal 525 for aphase detector, such as the phase detector 200 shown in FIG. 2, having ahysteresis adjuster and a filter, such as the hysteresis adjuster 300and the digital filter 320 shown in FIG. 3. A simulated output signal530 for a phase detector core, such as the phase detector core 310 shownin FIG. 3, is also shown.

By comparing the simulated signals shown in FIGS. 5A and 5B, persons ofordinary skill in the art will appreciate that undesirable jitter in thesimulated output signal 505 produced by the phase detector 200 that doesnot include a hysteresis adjuster 300 and a filter 320 is reduced in thesimulated output signal 525 produced by the phase detector 200 havingthe hysteresis adjuster 300 and the filter 320.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope and spirit of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

1. An apparatus, comprising: a phase detector adapted to determine aphase difference between at least two input signals; a first circuitadapted to generate a control signal based upon the determined phasedifference; and a second circuit adapted to: receive a first signal;receive a second signal; modify the second signal based upon the controlsignal; and provide the first signal and the modified second signal asthe input signals to the phase detector.
 2. The apparatus of claim 1,wherein the second circuit is adapted to modify the first signal beforeproviding the modified first signal to the phase detector.
 3. Theapparatus of claim 2, wherein the second circuit is adapted to providethe modified first signal and the modified second signal as inputsignals to the phase detector, and wherein the phase detector is adaptedto determine a phase difference between the modified first signal andthe modified second signal.
 4. The apparatus of claim 1, wherein thesecond circuit comprises a fixed delay and an adjustable delay.
 5. Theapparatus of claim 4, wherein the fixed delay comprises at least onedelay element.
 6. The apparatus of claim 4, wherein the adjustable delaycomprises at least two delay elements, at least one of the delayelements in the adjustable delay being selectable based upon the controlsignal.
 7. The apparatus of claim 1, wherein the phase detector iscapable of providing a signal indicative of the determined phasedifference to the first circuit.
 8. The apparatus of claim 7, whereinthe signal indicative of the determined phase difference is a binarysignal.
 9. The apparatus of claim 8, wherein the control signal is abinary control signal formed using the binary phase difference signal.10. The apparatus of claim 1, wherein the first circuit is adapted toprovide a signal indicative of a desired clock signal delay based uponthe determined phase difference.
 11. The apparatus of claim 1, whereinthe first circuit is a majority filter adapted to provide a signalindicative of the desired clock signal delay in response to at least twoconsecutive determined phase differences in the same direction.
 12. Theapparatus of claim 1, wherein the phase detector is at least one of alatch-type detector, an arbiter-type detector, and a counter-typedetector.
 13. The apparatus of claim 1, wherein the second circuitcomprises a hysteresis adjuster.